An exhaust gas recirculation apparatus (EGR) is known in the art which is disposed in a gas passage communicating between an air supply pipe and an exhaust pipe of an engine for an automobile to open or close the gas passage as required. To serve as a flow rate control valve for such EGR, one constructed as a poppet valve type is known (for example, see Japanese Laid-Open Patent Application No. 2002-54512).
A flow rate control valve for an exhaust brake unit is also known in the art which is of a butterfly valve type (see Japanese Laid-Open Utility Model Application No. 90523/1979 and Laid-Open Patent Application No. 151243/1995, for example), and is constructed as follows: in the arrangement of above Utility Model Application No. 90523/1979, a flow path in the exhaust pipe is formed at a given position with oppositely directed stepped end faces to provide seats, and a valve body is rotatably mounted adjacent to the stepped end faces which serve as seats. When required, the valve body is rotated through 90° to bring one end face of the valve body to be seated on one of the stepped end faces (seats) and the other end face of the valve body to be seated on the other stepped end face (seat), thus closing the flow path within the exhaust pipe.
It is noted that in the flow rate control valve of poppet valve type of the prior art as mentioned above, the disposition of the valve body within a gas passage involves a disadvantage that the flow rate of the exhaust gas passing through the gas passage is insufficient.
On the other hand, in the flow rate control valve of butterfly valve type mentioned above, the both stepped end faces which provide seats are formed at locations which are displaced from each other by an amount corresponding to the thickness of the valve body as viewed in the direction of flow of the exhaust gas and are directed in opposite directions.
Accordingly, in the conventional arrangement mentioned above, it is necessary to manufacture the stepped end faces which provide seats with a high accuracy in respect of the position where they are located, and it is also necessary to finish the surfaces of the both stepped end faces which provide seats with a high accuracy.
In addition, in the event a slight displacement occurs in the positions of the both end faces which provide seats, when the valve body is rotated through 90° to close the exhaust pipe in the manner mentioned above, a slight clearance is created between the end of faces of the valve body and the both stepped end faces which serve as valve seats, disadvantageously allowing a leakage of the exhaust gas therethrough.